Polysilicon is a raw material for semiconductor devices, solar cell devices, and the like, and the demand thereof is increasing recently. Conventionally, various methods for producing silicon which is a raw material for a semiconductor or a solar cell have been known, and some of them have already been carried out industrially.
Most commonly used high purity polysilicon is produced by chemical vapor deposition. Specifically, it can be produced by reacting trichlorosilane gas with a reducing gas such as hydrogen gas, as shown in Reaction formula 1 below.SiHCl3+H2→Si+3HCl  [Reaction Formula 1]
An example of commercially available methods is the Siemens method. FIG. 1 is a schematic view of an apparatus for producing polysilicon according to a conventional Siemens method. As shown in FIG. 1, in the polysilicon manufacturing apparatus according to the Siemens process, a silicon rod 12 is provided inside a vertical reaction tube 11, and the end of the silicon rod 12 is connected to the electrode 13. In addition, a gas supply nozzle 14 for supplying a reaction gas, trichlorosilane gas and hydrogen gas, is provided inside the reaction tube.
A method of forming polysilicon using the Siemens reaction tube constructed as described above will be described below. First, a current is supplied to the silicon rod 12 through the electrode 13, and the reaction gas is supplied into the reaction tube through the gas supply nozzle 14. The silicon rod 12 is heated by the supplied electric power to a surface temperature of about 1000 to 1150 □ and the reaction gas is pyrolyzed on the surface of the heated silicon rod 12, and thereby a high purity polysilicon is deposited on the silicon rod 12.
However, such a conventional Siemens reaction tube consumes a large amount of electrical energy, typically about 65 to 200 KWh/kg, and the cost for such electrical energy accounts for a very large portion of the cost of manufacturing the polysilicon. Further, since the precipitation is a batch type, there is a problem that an extremely complicated process such as installation of a silicon rod, electrification heating, precipitation, cooling, extraction, and vertical reaction tube cleaning must be performed.
Another method is the precipitation by using a fluidized bed. This method continuously produces silicon grains of 1 to 2 mm by supplying silane type material while supplying fine grains of about 100 microns as precipitation nuclei using a fluidized bed and precipitating silicon on silicon fine particles. This method has an advantage that it can be operated continuously for a relatively long period of time. However, since monosilane having a low precipitation temperature is used as a raw material for silicon, generation of fine particles of silicon by thermal decomposition of monosilane and precipitation of silicon onto the reaction tube wall are likely to occur even at a relatively low temperature, and thereby periodic cleaning or replacement of the reaction vessel is necessary.
Also, in Japanese Patent Laid-Open Publication No. H11-314996, it is disclosed a method using an apparatus having an exothermic solid, a high-frequency coil disposed to face a lower surface of the exothermic solid and at least one gas ejection port provided on the coil surface, and ejecting a raw gas containing a precipitation component at a lower surface of the heating body induction-heated by the high-frequency coil from the gas ejection port to perform precipitation and dissolution of the precipitation component on the lower surface of the heating body, and then producing a crystalline, for example, polycrystalline silicon, by dripping or flowing out a molten liquid from the bottom of the exothermic solid. However, according to said method, since the high-frequency coil and the exothermic solid are close to each other, the high-frequency coil requiring water cooling needs to take heat to maintain its function, resulting in low energy efficiency.
On the other hand, FIG. 2 discloses another apparatus for producing a polysilicon, in which a heating body which is a silicon deposition surface is formed into a tubular shape and the thermal efficiency is increased (see Korean Patent Publication No. 10-0692444). The apparatus comprises (a) a tubular container 21 having an opening serving as a silicon outlet at the lower end, (b) a heating device 23 for heating the inner wall from the lower end of the tubular container to an arbitrary height to a temperature equal to or higher than the melting point of silicon, (c) a chlorosilane supply pipe 25 comprising an inner tube having an outer diameter smaller than the inner diameter of the tubular container 21, one end of the inner tube being installed facing down in a space 24 surrounded by the inner wall heated to a temperature not lower than the melting point of silicon, and (d) a first sealing gas supply pipe 27 for supplying a sealing gas to the gap formed by the inner wall of the tubular container 21 and the outer wall of the chlorosilane supply pipe 25, and, in some cases, further comprise (e) a hydrogen supply pipe for supplying hydrogen gas into the tubular container. In said patent, the heating device 25 is described to be usable as a device capable of heating to a temperature higher than the melting point of silicon, that is, 1414° C. or higher, and a heating device using a high frequency, a heating device using a heating wire, and a heating device using an infrared ray are taken as an example.
Meanwhile, Japanese Patent Publication No. 4743730 discloses a method of manufacturing a silicon thin film by thermal plasma CVD. The Japanese patent discloses that by generating an electrothermal plasma by a complex plasma in which a high-frequency (RF) plasma is superimposed on a DC plasma, a deficiency of each plasma is supplemented to have a synergistic effect. However, in the DC plasma, a metal electrode must be inserted into the reaction vessel and this electrode must be in direct contact with the plasma. The electrodes exposed to the plasma gradually deteriorate and may cause impurities to be mixed.
Also, in the conventional vertical reaction tube, the polysilicon is produced on the surface of the reaction tube and is the produced polysilicon is melted, and then the polysilicon is collected from the lower collecting section. In this case, the raw gas supplied in the vertical direction passes through the vertically-shaped reaction tube without proceeding to the precipitation reaction at a high temperature in contact with the surface of the vertically-shaped reaction tube, resulting in lowering of production efficiency and lowering of energy efficiency. The polysilicon manufacturing process essentially comprises a vent gas collecting (VGR) process for collecting and recycling unreacted raw gas, hydrogen, and hydrogen chloride, which is a reaction byproduct, from the gas discharged from the reaction chamber. For example, hydrocarbons or unreacted chlorosilanes can be reused for the silicon deposition or precipitation process after being collected, and hydrogen chloride can be reused for the chlorosilane synthesis process. For example, in an actual process, conventional Siemens apparatus use two VGR systems per two deposition units.